1. Field of the Invention
The present invention relates to a beam-down type solar ray lighting device. More specifically, the present invention relates to a solar ray lighting device which is capable of reducing the occurrence of the blocking and the shadowing of the beams of light reflected by heliostats and which is also capable of supporting a center reflector stably and firmly.
2. Description of Related Art
Recently, there has been an increase in interest in the global environments such as: air pollution caused by exhaust gas produced by the combustion of fossil fuels; and the depletion of fossil fuels. In addition, alternative energy that may replace the aforementioned fossil fuels has attracted more public attention. For such alternative energy, wind power generation and photovoltaic power generation have been spreading.
Meanwhile, there is a concentrating-type solar thermal electric power generation system in which a heat-transfer medium is heated by use of heat produced by concentrating solar rays, steam is produced by the heat of the heat-transfer medium, a steam turbine is driven by the steam, and consequently electric power is generated. The system has attracted public attention because the system can be operated with similar power-generating facilities to those for the conventional thermal power station and can achieve a high output level.
Various types of concentrating-type solar thermal electric power generation systems have been proposed thus far, including a trough-type solar thermal electric power generation system (see, for example, Patent Document 1), a tower-type solar thermal electric power generation system (see, for example, Patent Document 2), and a dish-type solar thermal electric power generation system (see, for example, Patent Document 3). The trough-type system includes: reflectors each having a semi-circular sectional shape and having a light-reflecting surface formed in one surface thereof; and pipes extending in the axial directions of the respective reflectors, and a heat-transfer medium is introduced into the pipes. The tower-type system includes: a tower placed at the center and provided with a heat-transfer-medium heating portion on a top portion thereof; and multiple heliostats placed around the tower. The dish-type system includes: a bowl-shaped reflector having a light-reflecting surface formed in one surface thereof; and a heat-transfer-medium heating portion provided near the reflector.
Meanwhile, the reflector of the trough-type solar thermal electric power generation system has quite a large dimension in the width direction of the reflector. Since the reflectors are installed in lengthwise and widthwise, there is a problem that the trough-type system entails massive-scale installation.
The tower-type solar thermal electric power generation system is capable of increasing the light concentration in a relatively easy way simply by increasing the number of heliostats to be provided. Nevertheless, the tower-type system has its own problems. Firstly, the heat-transfer medium, such as a molten salt, is supplied to and is circulated through the heat-transfer-medium heating portion provided on the upper-end side of the tower. Accordingly, there is a problem that, at night when no solar rays are available, the tower-type system must keep the temperature of the molten salt by use of heating means such as an electric heater so as to prevent the molten salt from solidifying. In addition, the piping system for the molten salt becomes so long that the temperature of the molten salt is lowered, resulting in lower power generating efficiency of the tower-type system.
The dish-type solar thermal electric power generation system is a compact-sized system because the heat-transfer medium is heated by collecting the solar rays for each of the reflectors. There is a problem that the dish-type system, however, is not appropriate for massive-scale electric-power generation.
A system known as a beam-down solar thermal electric power generation system has been proposed as a different system from the above-described concentrating-type solar thermal electric power generation systems (see, for example, Non-Patent Document 1).    Patent Document 1: WO2005/017421    Patent Document 2: Japanese patent application Kokai publication No. 2005-106432.    Patent Document 3: Japanese patent application Kokai publication No. 2004-169059.    Non-Patent Document 1: Solar Energy, Volume 62, Number 2, February 1998, pp. 121-129(9)
As FIG. 18 shows, the beam-down solar thermal electric power generation system includes: three supporting posts 51a, 51b and 51c that stand vertically and have a truss structure; and a disc-shaped center reflector 55 (central reflector mirror) that is supported by the posts 51a, 51b and 51c. No reinforcing members are allowed to be placed between the supporting posts because such reinforcing members cause blocking and shadowing to take place. The central reflector mirror 55 sometimes has such a large diameter that exceeds 100 m. The central reflector mirror 55 has a structure with long intervals between every two supporting posts. Such a structure is made possible by combining structural pipes (multiple pipes with joint means). The central reflector mirror 55 sometimes has such a heavy weight that exceeds 3000 tons.
Accordingly, the supporting posts standing vertically without any reinforcing members provided in between have problems of an extremely low proof stress against the rotary force in the circumferential direction of the central reflector mirror, low resistibility against wind forces and low resistibility against lateral loads at the time of earthquakes. Moreover, the load on each of the supporting posts is large.
In addition, each supporting post is fixed at a fixing portion to the center reflector on the upper-end side and at an anchor portion on the bottom-end side of the supporting post. Such a structure can provide neither stability nor strength to the supporting posts. Accordingly, the power generating efficiency is eventually decreased by occurrence of problems concerning the safety, the life, and the offsetting of optical axis caused by the distortion or the incorrect positioning of the center reflector.
Furthermore, there is another problem. When the number of the supporting posts is increased in order to improve the strength, the blocking and the shadowing may occur, and thus the power generating efficiency is decreased. For this reason, the strength cannot be improved.